Injection of H3-leucine into the lumbar 7th dorsal root ganglia of cats is followed by a crest of protein-incorporated activity in the sciatic nerve with a linear rate of outflow close to 410 plus or minus 50 (S.D.) mm/day. Fast axoplasmic transport is independent of myelinated fiber diameter, and will be determined in nonmyelinated fibers using EM autoradiography and in various peripheral nerves and CNS tracts. A 'transport filament' hypothesis for the underlying mechanism was advanced with the requisite energy supplied by ATP derived from oxidative metabolism. Metabolic blocking agents were used to show this in in vitro preparations and this is to be further studied using 2- deoxyglucose (2-DG) to block glycolysis and fluoracetate (FA) to block the tricarboxyclic acid cycle. The level of approximately P (ATP plus CP) is to be related to the differing times of block of fast transport found with these agents. An Mg2-Ca2 ions activated ATPase present in mammalian nerve could utilize the ATP to supply the necessary energy for fast axoplasmic transport and its properties are to be investigated in analogy to muscle actomyosin. A Q10 of 2-2.3 was found for the decrease in rate of transport upon reducing temperature. Changes in the slope of the front of activity with reduced temperature and a cold block at temperatures between 0 to 8 degrees C are to be studied. Colchicine and vinblastine will be studied in in vitro preparations where a differentiation can be made of a complete or a partial blocking action on microtubules as well as reversibility. A common pool of ATP needed to maintain electrical excitability and fast axoplasmic transport will be studied in vitro by the recovery of these two functions from various durations of anoxia as well as the effects of increased O2 and CO2 pressures.